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Experimental Design

Maximum active layer thaw depths were measured annually in firelines, burned, and unburned control sites on the east and west sides of Cushman Creek. On the east side of Cushman Creek, three study sites were established: an unburned black spruce stand (2130), a heavily burned black spruce stand (2004), and a fireline (2012) between the unburned and burned stands where vegetation and organic material were removed by bulldozer down to mineral soil. At each site on the east side, ten permanent probe points (marked with fiberglass poles) at 2 m intervals were established on a transect perpendicular to the fireline. On the west side of Cushman Creek, four study sites were established: an unburned black spruce stand (2003), a lightly burned black spruce stand (2001), a heavily burned black spruce stand (2002), and a fireline (2010) between the heavily and lightly burned stands. The west side fireline transect was established similar to the east side sites (10 probe points at 2 m intervals, perpendicular to the fireline). In the west side unburned control, heavy burn, and light burn sites, stakes from 20 permanent vegetation plots were used as probing points (10 to 20 plots were probed). The vegetation plots were spaced at 30 m intervals, usually in 4 rows of 5 plots. Burn severity was evaluated as lightly burned (2001) if less than 50 percent of the ground surface was blackened, the lesser vegetation was not consumed, branchlets were still present on trees, and tree crowns were scorched but not consumed. Burn severity was evaluated as heavily burned (2002 & 2004) if more than 90 percent of the ground surface was blackened, and the lesser vegetation and tree crowns were consumed (Viereck and Dyrness, 1979). Using more recent standards of burn severity, the heavily burned stands (2002 & 2004) would be classified as ?moderately burned?, with a partially consumed organic layer, shallow ash layer present, but with woody twig parts remaining (Viereck et al. 1979). Due to the long-term nature of this study, precise probing locations within each site were inconsistent (especially in the early years) and were occasionally changed due to formation of runoff gullies, ditches, or recent machinery-caused disturbance. See corrections/comments headings and site maps for a detailed history of probing locations in each site and the most recent plot/pole labels and locations for future sampling.

Methods

Active layer thaw depths at each probing location were measured annually in mid to late September near the time when maximum thaw depth is reached. At each probing location, a metal frost probe (with extensions if needed) was inserted into the soil until it reached the hard frozen ground of the permafrost, and depth was recorded. Depth to permafrost = active layer thaw depth. All measurements are in cm units.